The present invention relates to methods of casting metal in a rotary mould or die, of the kind in which protection is provided simultaneously for the surface of the bath of metal and the space within the mould, prior to casting, by means of a controlled stream of liquefied inert gas which on the one hand is directed over the surface of the bath and which on the other hand is fed into the interior of the mould as it is driven in rotation. Hereinafter, such a method will be referred to as "of the kind described".
This method is applicable to the manufacture of centrifugally cast tubes. Before the metal is introduced into the mould or die, there is first introduced into it a sufficient quantity of liquefied inert gas to give an inert period which will allow the time required to perform the casting proper. With this manufacturing technique, which presupposes that metal is cast only once, the liquefied gas is only introduced prior to the casting.
It is an object of the invention to enable parts formed from a plurality of metals, which thus require a plurality of successive casting operations in the rotary die, to be manufactured.
There are many metal parts which being bodies of revolution, are produced by moulding in a rotary mould or die and which, to withstand the conditions under which they are forced to operate, need to exhibit certain properties to a vary pronounced degree. Thus, ducts intended for conveying corrosive liquids and pipes intended to be submerged in the sea need to exhibit a high resistance to corrosion, whereas rolls for rolling mills need to have circumferential surfaces of extreme hardness. However, metals or alloys which have the requisite qualities are extremely costly and this results in their being used to form only a part of the final article to be obtained, the remainder of the said article being formed from an ordinary, less costly metal. In the case of a duct intended to convey a corrosive liquid for example, only the inside wall which is in direct contact with the said liquids needs to exhibit high resistance to corrosion, to a depth which may be small, and the remainder of the wall, which simply provides mechanical strength, may be formed from ordinary cast iron or steel. Similarly, in the case of a rolling-mill roll only the body, that is to say the outer working surface, needs to be extremely hard, while there is no need for the core of the roll to be of extreme hardness and indeed it is preferable for it to exhibit other characteristics, such as a certain amount of resilience, which leads to its being produced from a relatively malleable and inexpensive material such as grey cast iron.
To produce such articles it is necessary to cast a plurality of metals, generally two, of different kinds in succession in the rotary mould or die, and thus to perform the moulding in a plurality of stages. The first casting operation allows a first layer to be obtained and a second casting operation, which is performed after the time required for the first layer to solidify, allows a second layer to be obtained, and so on.
This procedure has many drawbacks. The mould or die, which is driven in rotation and which is normally preheated, forms a hot, open enclosure which sucks in the surrounding air vigorously. The result is that the casting and solidification of the layers of metal take place in an enclosure through which a stream of very hot and turbulent air flows. When a layer has solidified under these conditions it becomes oxidised on its inner face and this prevents the layer of metal added by the next casting operation from adhering properly. The bond between the two layers is seriously defective and this detracts from the quality of the finished article.
It is therefore a further object of the invention to overcome or minimise these disadvantages and to enable this object to be achieved.